1. Field of the invention
This invention relates to a pin tractor for feeding blank paper for use in various printers, typewriters, etc., which are used as output devices for electronic computers, especially for personal computers.
2. Description of the prior art
In various printers, typewriters, etc. to be used as output devices for electronic computers, etc., pin tractors which feed blank paper by engaging pins of an endless belt carrying pins with feed perforations at both edges of blank paper and by running the endless belt carrying pins, are generally used. Methods of driving the belt carrying pins of such pin tractors are roughly divided into:
(i) the driving is effected by a driving toothed pulley and a driven toothed pulley (or flat pulley), as disclosed by U.S. Pat. No. 4,130,230, for example, and
(ii) the driving is effected by a driving toothed pulley and a guide part having a semicircular portion, as disclosed by U.S. Pat. No. 4,315,585 and Japanese Utility Model Registration Application Laying Open Gazette No. 57-135537.
However, according to the driving method of item (i) above, a driving torque (turning torque) of the belt carrying pins becomes small but the measurement of the pin tractor as a whole becomes larger, due mainly to the space required for the arrangement of a fixing mechanism which fixes a pin tractor to a guide shaft. Thus, compactness of the pin tractor cannot be expected from this method.
According to the driving method of item (ii) above, the pin tractor as a whole can be compacted to some extent but turning torque of the belt carrying pins becomes larger, for which it is necessary to make the driving load of the belt carrying pins larger.
With the recent development of miniaturization of printers, etc. and the resultant space saving, miniaturization of pin tractors has been demanded. It has also been desired in the field of printers to lessen the turning torque of the belt carrying pins from the aspect of saving power consumption. For example, as disclosed by U.S. Pat. No. 3,825,162, it has been known to form a teeth portion of the belt carrying pins in semispherical shape but even with this, turning torque could not be reduced to the desired degree. In such conventional pin tractors, the belt carrying pins is mounted rather loosely so as to reduce the turning torque of the driving pulley and is adapted to run straight in a guide groove of open cut shape (refer to U.S. Pat. No. 3,825,162, for example). However, since the guide groove has guide surfaces close to both side edges of the belt carrying pins and the belt carrying pins runs in such a fashion that the blank paper feeding surface of the belt (surface of the belt base) becomes almost the same level with the surface of the frame, blank paper feeding load occurs due to the slide resistance between blank paper (at feeding) and the surface of the frame, with resultant lowering of blank paper feeding function. This tendency is especially conspicuous in the case of blank paper of rough quality. Also, if the belt carrying pins is loose, when blank paper is fed by feeding at a loose side of the belt, the belt rises up considerably and as a result, accuracy of paper feeding lowers.
As disclosed by U.S. Pat. No. 4,214,691 and Japanese Patent Application Publication Gazette No. 57-17434, for examples, the conventional pin tractor is so designed that it is supported movably by a guide shaft and a driving shaft and is adjustable in position according to the width of paper to be fed. A lock means is provided so that the pin tractor can be fixed at the desired position in relation to the guide shaft.
In the conventional lock means mentioned above, fixing to and releasing from the guide shaft depend upon the revolving direction of an operational lever and therefore it is necessary to revolve the operational lever in the same direction at all times for fixing the pin tractor to the guide shaft. In view of this necessity, different pin tractors are used to the left side and to the right side of paper. This poses a problem involving the requirement of more parts and higher manufacturing costs. In this connection, a lock means which can carry out fixing to and releasing from the guide shaft, irrespective of the revolving direction of the operational lever in relation to the fixing member, is available (U.S. Pat. No. 4,614,287, for example) but as this lock means comprises a fixing member and an operational lever, it costs higher from the standpoint of formation of parts and more processes for construction.